1. Field of the Invention
The present invention relates to an electronic telephone apparatus for controlling the switching of telephone lines, and more particularly to an electronic telephone apparatus which can reliably supply sync clock signals, and also to a method of controlling the supply of sync clock signals.
2. Description of the Prior Art
An electronic telephone apparatus is designed to switch telephone lines. An typical example is disclosed in U.S. Pat. No. 4,802,209 (Jan. 31, 1989). Generally, the electronic telephone apparatus has line circuits for connecting telephones, a control unit, trunk circuits, a memory, a tone signal source unit, and an exchanging switch. Telephones are connected to the line circuits. The line circuits are interfaces, each assigned to a specific telephone number. The line circuits function as connection interfaces, connecting themselves with the exchanging switch. The trunk circuits function as interfaces between the exchanging switch and a station line. The exchanging switch performs switching between the trunk circuits, on the one hand, and the line circuits, on the other, and also performs switching among the line circuits. The control unit controls the exchanging switch in accordance with the dial data input through the trunk circuits or the line circuits. It also controls the other components of the apparatus. The memory stores various programs and various tables which the control unit executes and refers to, in order to perform switching, various tables; it also stores data representing the functions set to the apparatus. The tone signal source unit is designed to generate various tone signals informing of the conditions of the apparatus, such as ringing-tone signals and busy-tone signals.
When any telephone connected to one of the line circuits generates an off-hook signal, this line circuit supplies the off-hook signal to the control unit. Then, the line circuit transmits the dial data supplied from the telephone to the control unit. The control unit determines the receiving telephone from the dial data, and controls the exchanging switch such that a telephone line is connected to the line circuit or the trunk circuit which is connected to the receiving telephone. When the control unit receives a confirmation signal from the trunk circuit, it supplies a ringing-tone signal to the line circuit specified by the dial data or the like contained in the confirmation signal, or to the line circuit specified by the data stored in the memory, unless the telephone connected to the line circuit is busy. As a result, the receiving telephone rings. When the receiving telephone is uncradled by the respondent, the control unit controls the exchanging switch such that the line circuit connected to the receiving telephone is connected to the trunk circuit. Hence, the sender and the respondent can talk through the telephone.
The exchanging switches recently developed comprise time division switches (hereinafter referred to as TSWs) that are designed to perform time-division multiplex transmission of digital data. An input line and an output line are connected to a TSW. These lines are highways for transmitting PCM (Pulse Code Modulated) signals. (Hereinafter, the input line and the output line will be called "input PCM highway" and "output PCM highway," respectively.) A PCM codec for converting speech signals to PCM codes is connected to both the input PCM highway and the output PCM highway. Further, a trunk circuit is connected to the input PCM highway and the output PCM highway. This trunk circuit connects both PCM highways to a telephone network. A telephone is connected to the PCM codec by a line control circuit which functions as an interface.
Fundamental dyadic telephone talk is achieved in the following way. First, a sender takes up the receiver of the telephone and dials the number of the receiving telephone. As a result, the calling telephone generates an off-hook signal and a dial signal. These signals are detected by means of the loop monitoring performed by the line control circuit, and are output as serial data to the data highway of a control unit under the control of the control unit. The data on the data highway is input to a CPU circuit which performs various operations and various controls. The CPU circuit analyzes the dial signal in accordance with a program, thereby providing data for generating a ring-tone signal which is to be supplied to the receiving telephone of the number specified by the dial signal. This data is output to the data highway and supplied to the line control circuit. The control circuit generates a ring-tone signal from the input data. The ring-tone signal is transmitted to the receiving telephone. When the receiving telephone makes a response, the CPU circuit gives a command via the control circuit to the codecs coupled to the calling and receiving telephones, thus activating these codecs so that the sender and the respondent can talk through the telephone. Either codec is on standby until it receives the command. In other words, it becomes active upon receipt of the command and performs AD/DA (Analog-to-Digital, Digital-to-Analog) conversion of speech data.
The CPU circuit performs the exchanging of speech data between the calling telephone and the receiving telephone. It also controls the TSWs in order to achieve time-division multiplex transmission of the speech data. The PCM codes supplied from the codecs of the calling and receiving telephones are input to one of the TSWs via a receiving PCM highway PCM IN. The TSWs exchanges the PCM codes. The PCM codes, thus exchanged, are output to a receiving PCM highway PCM OUT. In most telephone systems, PCM data for 32 channels is multiplexed on each PCM highway. From a viewpoint of time series, the PCM data has 32 time slots. For instance, the PCM data of the first telephone is assigned to the first time slot, that of the second telephone to the second time slot, and so forth. To accomplish a dyadic talk between the first telephone and the second telephone, it suffices to exchange the PCM data items of these telephones with each other.
To achieve this exchange of PCM data items, to drive the TSWs thereby to control the time slots, and to drive the PCM codecs, the control circuit, and the like, a drive clock signal is required. This clock signal is generated from the clock pulses output by an oscillator circuit if the electronic telephone apparatus is used in combination with a telephone network which comprises analog lines.
On the other hand, if the electronic telephone apparatus is used in combination with a telephone network which comprises digital lines, the drive clock signal is generated from the sync clock signal supplied via these digital lines, so that the electronic telephone apparatus can accurately synchronize the data being transmitted by the digital lines.
In most cases, a PCM codec outputs a PCM code every 125 microseconds. Hence, data of 2.048 Mb is transmitted every section via a PCM highway. Therefore, a 2.048 MHz clock signal is used to drive the codec. To synchronize the data transmitted at such a high speed through digital lines, the electronic telephone apparatus uses the drive clock signal generated from the sync clock signal supplied through the digital lines. The sync clock signal also is supplied via the trunk circuit. The trunk circuit is usually mounted on a printed circuit board which is incorporated in the electronic telephone apparatus. This arrangement may cause problems. It is possible that the connector connecting the printed circuit board to the other components of the telephone apparatus fails to function. In addition, if the print circuit board is disconnected from the telephone apparatus for inspection, the sync clock signal is no longer supplied through the trunk circuit. Consequently, the electronic telephone apparatus stops functioning, and telephone talks can no longer be achieved.